Seattle “During decades, scientists believed that Prochlorococcus, the smallest and most abundant phytoplankton on earth, would prosper in a warmer world.” But new investigation suggests that the microscopic bacteria, which forms the base of the Marina Food Network and helps regulate the climate of the planet, will decrease dramatically as the seas are hot.
A study published Monday in Nature Microbiology found that procchlorococcus populations could be reduced by up to 50% in tropical oceans in the next 75 years if the surface waters exceed 27.8 ° C (82 ° F). Many temperatures of the surface of the tropical and subtropical sea are already tending above the average and is projected that they will regularly exceed 30 ° C (86 ° F) during that same period.
“These are key species, very important,” said François Ribalet, associate professor of research at the Oceanography School of the University of Washington and main author of the study. “And when a key species decreases in abundance, it always has consequences in ecology and biodiversity. The food network will change.”
These little organisms play a vital role in oceanic life. Prochlorococcus inhabits up to 75% of the surface waters illuminated by the Earth’s sun and produces approximately one fifth of the oxygen of the planet through photosynthesis. More crucially, Ribalet said, they convert sunlight and carbon dioxide into food at the base of the marine ecosystem.
“In the tropical ocean, almost half of the foods are produced by Prochlorococcus,” he said. “Hundreds of species depend on these types.”
Although other forms of phytoplankton can move and help compensate the loss of oxygen and food, Ribalet warned that they are not perfect substitutes. “Evolution has done this very specific interaction,” he said. “Obviously, this will have an impact on this unique system that has been established.”
The findings challenge decades of assumptions that prochlorococcus would prosper as the waters warm up. These predictions, however, were based on limited data of laboratory crops. For this study, Ribalet and his team tested water samples while crossing the Pacific over the course of a decade.
During more than 100 research cruises, the equivalent of six trips around the world, counted about 800,000 million individual cells taken from samples at each kilometer. In his Laboratory of the University of Washington, Ribalet demonstrated the Seaflow, a box full of tubes, cables and a penetrating blue laser. The tailored device continuously extracts seawater, which allowed the equipment to count the microbes in real time. “We have counted more prochlorococcus than there are stars on the Milky Way,” said Ribalet.
Paul Berube, a research scientist at the Massachusetts Institute who studies Prochlorococcus but did not participate in work, said the amplitude of the data is “innovative.” And he said that the results fit with what is known about the simplified genome of the microbe, which makes it less adaptable to the rapid environmental changes.
“They are at the very base of the food network and feed everything else: the fish eat the things that eat the phytoplankton and we eat the fish,” he said. “When changes are being made on the planet that influence these particular organisms that are essentially feeding us, that will have great consequences.”
To test if Prochlorococcus could evolve to resist warmer conditions, the Ribalet team modeled a hypothetical heat tolerant strain, but discovered that even those “would not be enough to completely resist the warmest temperature if greenhouse gas emissions continue to increase,” Ribalet said.
He stressed that study projections are conservative and do not take into account the impacts of plastic pollution or other ecological stressful factors. “Actually, we try to present the best case,” said Ribalet. “Actually, things can be worse.”
Steven Biller, Wellesley College’s associated professor, said the projected decreases are “scary but plausible.” He pointed out that Prochlorococcus is part of the “invisible forests” of the ocean: small organisms in which most people never think, but are essential for human survival.
“Half of all photosynthesis is happening in the oceans and prochlorococcus is a really important part of that,” said Biller. “The magnitude of the potential impact is surprising.”
Biller, Berube and Ribalet said that while other microbes can compensate for something, the broader risks for biodiversity and fishing are real.
“We know what global warming promotes. There is no debate within the scientific community,” said Ribalet. “We need to stop greenhouse gas emissions.”
He expects the findings to attract more attention to tropical oceans, which could serve as natural laboratories for warming adaptations and as early warning signs for ecological collapse.
“For the first time, I want to be wrong. I would love to be wrong,” he said. “But these are data based on data.”